1. Field of the Invention
The present invention relates to an optical scanning apparatus such as a laser beam printer constructing an output part of a copy machine, a facsimile, and a computer or the like.
2. Description of the Related Art
In a copy machine and a printer for scanning a laser beam by a rotary polygon mirror which is a rotary polyhedral mirror and forming a color image, an electrostatic latent image is formed on a photosensitive drum for each color by the laser beam and the electrostatic latent images are developed by a plurality of development units for each color. Then, toner images made of a single color such as Yellow (hereinafter, abbreviated as Y), Magenta (hereinafter, abbreviated as M), Cyan (hereinafter, abbreviated as C), and Black (hereinafter, abbreviated as K) are formed. Then, transferring them on an intermediate transfer member and superimposing them, respectively, a color image is formed.
Next, this will be briefly described with reference to FIG. 5. Reference numerals 101 to 104 denote drum units for forming color toner images of Y, M, C, and K on photosensitive drums, respectively. The toner image formed on each photosensitive drum is superimposed on a transfer belt 113 which is an intermediate transfer member. Then, an unfixed toner image is transferred on a recording material by transcription means, and the toner image is fixed on the recording material due to heat by fixing means.
As such a color image forming apparatus, there are two types, namely, a single drum system for forming a toner image on a single photosensitive drum more than once and a so-called tandem drum system for arranging a plurality of photosensitive drums corresponding to each color and transferring the toner images collectively on a recording paper or the intermediate transfer member.
In any type of color image forming apparatuses, unless a print position of each color such as Y, M, C, and K is correctly aligned with the recording paper, color shading or the like due to color drift is generated so as to significantly degrade an image quality.
There are various causes of color drift, however, limiting the cause to the color drift of exposure due to a laser scanning apparatus, deviation of an irradiation position due to variation of an ambient temperature and deviation of a writing position in a sub-scanning direction (a recording paper feeding direction) due to curvature; of a scanning line and dispersion of a rotational phase of a rotary polygon mirror or the like may be considered.
As an art to decrease these color drifts of exposure, a method for correcting color drift by sequentially measuring the deviation of the irradiation position and the deviation due to curvature of the scanning line when the apparatus is operating, and moving or rotating a lens and a mirror in accordance with the deviation amount has been known. In addition, with respect to the color drift due to dispersion of the rotational phase of the rotary polygon mirror, various suggestions are made to control the rotational phase by adjusting a rotation speed of the rotary polygon mirror under PLL (Phase Locked Loop) control.
In Japanese Patent Application Laid-Open No. 9-104132, the mechanism for suppressing the phase deviation within a range of a predetermined value by recognizing the deviation of the phase with respect to the writing position from a signal synchronization signal of a main body of an image forming apparatus and a BD signal of a laser scanning apparatus and varying a frequency of a reference signal in accordance with the deviation is disclosed.
Thus, according to the art for preventing the color drift due to the rotational phase of the rotary polygon mirror, an image forming apparatus for scanning one laser beam for each rotary polygon mirror is more advantageous because the rotational phases of the rotary polygon mirror can be adjusted among respective lasers and the color drift can be suppressed within several microns on the recording paper. The same applies to the single drum system or the tandem drum system if one rotary polygon mirror is rotated with respect to one laser.
However, in the image forming apparatus in recent years, speeding up of the image output has been required, so that the above-described tandem drum system is becoming a mainstream. In addition, miniaturization and price reduction of the apparatus has been also strongly required and it is preferable that the laser scanning apparatus is not provided for each photosensitive drum in the tandem system. Therefore, as shown in FIG. 5, an image forming apparatus B configured as an all-in-one mechanism for scanning all photosensitive drums 101 to 104 by one rotary polygon mirror 110 has been also devised.
In such a laser scanning apparatus of the all-in-one mechanism, entering a plurality of laser beams in the rotary polygon mirror, each laser beam is scanned on the different photosensitive drum, so that it is not possible to match the rotational phases of the rotary polygon mirror among respective colors in principle.
As a result, the deviation caused by the phase of the rotary polygon mirror has been generated relatively within one scanning line at the maximum among respective laser beams. One scanning line is about 42 μm when a sub-scanning resolution is 600 dpi. Since the rough standard of the color drift which is allowed on the recording paper is about 100 μm, this is not an amount which can be ignored.
Therefore, according to the mechanism such that a plurality of laser scanning apparatuses for irradiating a plurality of laser beams is provided, the color drift can be decreased.
According to such a mechanism, in each laser scanning apparatus, it is possible to improve a scanning accuracy of the laser beam which is the standard of the rotational phase of the rotary polygon mirror. However, if the standard of the rotational phase of the rotary polygon mirror is made into the laser beam for forming a toner image with indistinctive brightness on the image in a plurality of laser scanning apparatuses, the scanning accuracy of the laser beam for forming a toner image with the low brightness which is highly visible oh the image is made lower. As a result, the color drift is highly visible on the image.
Accordingly, when providing a plurality of laser scanning apparatuses, it is preferable that the laser beam having major impact on the image is defined as the standard of control of the rotational phase of the rotary polyhedral mirror.